A thin film transistor-liquid crystal display (TFT-LCD) has become a mainstream product in the current flat-panel display market due to its advantages such as small volume, low power consumption and being free of radiation. There is a very strict requirement of a dust level in a production environment during the manufacture of the TFT-LCD, especially for processes before the packaging. This is because the product quality may be seriously affected by the dust. In order to reduce the dust level as well as a manipulation degree, a large number of automatic conveyance devices are used during the manufacture of the TFT-LCD, and meanwhile a robot arm is used to join the devices. However, in the case that a glass substrate is delivered by an upstream device erroneously, it may be located on the robot arm at an erroneous position, and thereby it may be damaged due to a collision with a downstream device.
To overcome this drawback, the robot arm is provided with an optical fiber sensor by some manufacturers, so as to sense positions of the glass substrate and the robot arm, thereby to determine a relative position of the glass substrate to the robot arm. However, there are also the following drawbacks.
1. In order to enable the glass substrate to be at a predetermined position on the robot arm, picking and placing actions may be repeated by the robot arm, so as to adjust the relative position of the glass substrate to the robot arm. This procedure is complex and time-consuming, so the tact time will be reduced.
2. This method may merely be used to detect deviations in a front-and-back direction as well as angular deviations, rather than deviations in left-and-right direction, so it is still impossible to prevent the occurrence of deviation completely.